The purpose of this study is to evaluate the protective efficacy of pneumococcal vaccine for the elderly and for other patients with indications for the vaccine. Although pneumococcal vaccine is currently recommended for many patients who have an increased risk for serious pneumococcal infections, the use of the vaccine is controversial because of uncertainty about its clinical efficacy. It is unlikely that randomized clinical trials will resolve this uncertainty because the low incidence of serious pneumococcal infections requires studies with a prohibitively large sample size to assure statistically meaningful results. In addition to the logistical and financial obstacles to conducting such trials, ti may be ethically unacceptable to administer a placebo rather than a licensed vaccine to patients at risk for life-threatening pneumococcal infections. The proposed research will employ the case-control method, a statistically powerful and ethically acceptable alternative method for assessing the vaccine's efficacy. The study subjects will be hospitalized patients who have previously recognized indications for pneumococcal vaccine. The cases will be patients with systemic pneumococcal infections and will be identified through active surveillance of the bacteriology laboratories of the participating hospitals. It is expected that two-third of these patients will be 55 or more years of age. The specimens of pneumococci from these patients will be saved and serotyped. One control will be randomly selected from patients without pneumococcal infection who are matched to each case by the indication for the vaccine (e.g. chronic lung disease), age, date of hospitalization, and the duration that the indication had been recognized. Antecedent receipt of pneumococcal vaccine will be ascertained by "blinded" research assistants using the inpatient and butpatient records of all sources of medical care. Matched odds ratios will be used to calculate both the overall effectiveness of the vaccine for all systemic pneumococcal infections and the vaccine's protective efficacy against systemic infections due only to serotypes of penumococci included in the vaccine. Logistic regression will be used to adjust for potential confounding variables and to assess the independent effect of age on the protection afforded by the vaccine. The study will provide statistically precise estimates of the vaccine's efficacy both for the overall sample and for specific subgroups of patients. This information should have a substantial impact on both national and individual preventive health-care planning.